Japanese Unexamined Patent Application Publication No. 7-233383 (Patent Document 1) has been disclosed a solid fuel production process for producing a solid fuel from a coal such as a low rank coal. In this process, an oil mixture containing a heavy oil and a solvent oil is mixed with a porous coal, which is equivalent to the low rank coal, to prepare a raw material slurry. The slurry is heated to dewater the porous coal so that the porous coal contains the oil mixture of the heavy oil and the solvent oil in the pores. Then, the slurry is subjected to solid-liquid separation. (The solid fuel production process disclosed in this patent document is hereinafter referred to as known process A.)
Known process A can produce a low-pyrophoricity transportable, storable dewatered solid fuel.
Since porous coal (low rank coal) contains a large proportion of moisture, the transport of the porous coal means that of moisture in a sense, and the transportation cost is relatively increased. Hence, porous coal is inferior in transportability. Also, the amount of calories is reduced by an amount equivalent to the moisture content. Accordingly, it is desired that porous coal be dewatered. Unfortunately, if the dewatering is performed by drying and evaporation using a tubular dryer or the like, oxygen adsorbs to and oxidizes the active points in the pores of the dewatered porous coal. Consequently, an accident by spontaneous combustion can occur undesirably.
Accordingly, known process A evaporates or vaporizes the moisture in the pores of the porous coal by heating the raw material slurry (mixed material containing a porous coal and an oil mixture of a heavy oil and a solvent oil) (thereby dewatering the porous coal), and allows the oil mixture containing the heavy oil to cover the pores. Thus, the oil mixture, particularly preferentially the heavy oil, finally fills the pores. Consequently, oxygen is inhibited from adsorbing to and oxidizing the active points in the pores and, thus preventing the spontaneous combustion. Thus, known process A can produce a low-pyrophoricity, transportable, storable dewatered solid fuel.
The above-cited patent document (Japanese Unexamined Patent Application Publication No. 7-233383) also discloses an apparatus for producing such a solid fuel. The solid fuel production apparatus includes a mixing vessel in which an oil mixture containing a heavy oil and a solvent oil is mixed with a porous coal to prepare a raw material slurry, an evaporator that heats the raw material slurry to remove the vapor, and a solid-liquid separator that performs solid-liquid separation on the heated and thus treated slurry. (The solid fuel production apparatus disclosed in this patent document is hereinafter referred to as known apparatus A.) The porous coal used in the apparatus is equivalent to the low rank coal, as above.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 7-233383